7.1 Chromatic Aberrations

Would a perfect optical surface have aberrations?

yes

What causes aberrations? (3 things)

way light is refracted by a particular surface

index of the medium

location of object point

Chromatic aberrations are dependent on?

lens material

Achromatic aberrations are dependent on?

lens design (lens form)

For most lens materials, how does the refractive index change for different wavelengths?

refractive index is higher for shorter (blue) wavelengths + lower for longer (red) wavelengths

For lenses, what focuses first, short (blue) wavelengths or long (red wavelengths)

blue focuses first

What is longitudinal chromatic aberration (LCA) [aka axial chromatic aberration]?

dioptric distance between the red and blue focus

How do you calculate longitudinal chromatic aberration (LCA)??

LCA=P/V

What is longitudinal chromatic aberration (LCA) dependent on?

Power of lens

Abbe Value

How does LCA change as lens power changes?

increasing lens power = increasing LCA

How does LCA change as abbe value changes?

decreased Abbe value= increased LCA

What clinical test makes use of the eyes longitudinal chromatic aberration?

bichrome test

What is transverse (lateral) chromatic aberration (TCA)?

difference in prismatic effects of the lens for different wavelengths

How do you calculate TCA?

prism power/abbe value

How does TCA change as abbe value changes?

decreased abbe value=increased TCA

How does TCA change as prismatic power changes?

TCA increases as prismatic power increases

What does TCA cause for a patient?

colored fringes around borders

blurred image

decreased performance

What is the standard unit for TCA?

prism diopters

What is the standard unit for LCA?

diopters

How can chromatic aberration be controlled? Where is this not used?

using a achromatic doublet

not used in spec lenses

How do we control for chromatic aberrations in specs?

use material with higher abbe value

frame PD=pts PD, center frame vertically

shorter vertex distance